Steam generators in pressurized water nuclear reactors comprise a casing of generally cylindrical shape, inside which a water container is delimited by a tube plate of great thickness, on which the bundle of steam generator tubes is fixed. Each of the tubes of the bundle comprises two parallel rectilinear branches, the end parts of which are fixed inside the tube plate, generally by expansion and by welding.
In a steam generator in operation, the tubes are in contact both with the primary fluid of the nuclear reactor, consisting of pressurized water, and with the secondary fluid consisting of feed water, which is vaporized. Under the operating conditions of the steam generator, the tubes are subject to deterioration of various kinds, particularly to corrosion by the fluids with which they are in contact. Some tubes may become defective in operation and, for example, develop cracks giving rise to leaks, which entail a contamination of the secondary fluid.
During reactor maintenance periods, it is therefore necessary to work on the steam generator and to close off the tubes showing deterioration capable of giving rise to leaks.
Various methods and devices are known which enable a plug to be placed in the end part of a steam generator tube from the water container of the steam generator.
Use is made in particular of closure plugs comprising a tubular body closed at one end by an end wall and provided on its outer surface with outwardly projecting splines assisting the attachment of the plug to the interior of the tube which it is desired to seal off. The plug, whose maximum outside diameter is slightly smaller than the inside diameter of the tube, is introduced through the end of the tube flush with the inlet face of the tube plate.
A clamp block inserted into the body of the plug is pushed or pulled inside the body, which is in position in the tube, in such a manner as to bring about the diametrical expansion of the plug body. The outer surface of the plug, provided with the splines, is thus sealingly applied against the inner surface of the tube, in such a manner as to achieve simultaneously the locking of the plug and the leaktight closure of the tube. The clamp block and the internal bore of the plug generally have corresponding frustoconical shapes permitting the diametrical expansion of the plug body.
This type of plug is generally called a mechanical plug, in contrast to a plug the clamping of which is achieved at least in part by means other than a clamp block movable axially in the bore in the body.
In the case of mechanical plugs, the clamp block remains in place inside the body in order to lock the plug.
In certain cases it may be necessary to remove a plug placed in position in a steam generator tube.
The tubes situated in the central part of the bundle have a curved portion with a small radius of curvature, connecting their two rectilinear branches. The portions having a small radius of curvature, known as small curves, make the tubes in question particularly subject to corrosion because of the relatively great concentration of relatively great of stresses in the curved portion.
As a preventive measure, series of operations have been undertaken for the sealing off of tubes in the central part of the bundle in certain steam generators.
Since then, methods and devices for the effective relief of stresses on the small curves have been developed, and it has become possible to put back into service tubes which had been sealed off as a preventive measure.
In addition, methods are also known for the effective repair of tubes, enabling tubes which had been sealed off to be put back into operation after repair.
It is therefore desirable to have available methods and devices making it possible to effect the extraction of plugs placed in steam generator tubes.
It has, for example, been proposed to lengthen the plug body so as to reduce its outside diameter by contraction, in order to enable the plug to be subsequently extracted. This elongation of the plug body can be effected by thrust applied to the closure end wall of the body, for example with the aid of the clamp block, which in its central part has a tapped hole in which a threaded rod is engaged. The threaded rod is in turn connected to a thrust device, such as a jack.
The disadvantage of this method is that it may lead to a rupture of the tubular plug body, thus entailing additional machining operations, with a consequent increase in irradiation doses for the operators who carry out these machining operations.
The reduction of the thickness of the wall of the plug body by drilling it to a desired diameter has also been considered. However, this drilling cannot extend to a zone close to the end wall of the plug, because the clamp block is driven rotationally through the action of the torque applied by the drill.
Consequently, it is not possible to effect unclamping of the plug by this method, and its extraction is accompanied by damage caused to the inside wall of the tube.